Electrodes in optoelectronic semiconductor waveguide devices are generally provided as metallizations. Current metallization schemes generally use simple structures. The principal component is gold, due to its high electrical and thermal conductivities, although further metal layers are usually located between the gold and the semiconductor to act as barriers to interdiffusion and excessive reaction of gold to the semiconductor.
For example, a typical metallization scheme for use with III-V semiconductors, such as InP, is Ti:Pt:Au (where X:Y:Z denotes successively deposited layers of materials X, Y and Z), each comprising a single layer of each metal. Each layer is polycrystalline, i.e. the metal forms into separate crystals.
FIG. 1 illustrates a such a typical prior art metallization 1 on an InP semiconductor substrate 2. An insulation layer 3 (e.g. silicon dioxide) and a metallization structure comprised of three metallization layers 4, 5 and 6 are formed on the substrate 2. The metallization layers serve different purposes and have different relative thicknesses:                A relatively thin titanium layer 4 provides a good adhesion to the semiconductor and blocks diffusion of indium out of the semiconductor.        A relatively thin intervening platinum layer 5 prevents diffusion of gold through the titanium and into the semiconductor, which would damage the semiconductor's performance.        A relatively thick gold layer 6 provides a low electrical resistance, a robust structure and a high rate of thermal dissipation. It is desirable that the gold layer should be sufficiently thick to provide a low electrical resistance for good current flow through the electrode, into the device beneath, and that it should also provide a high thermal conductivity for good thermal dissipation from the active regions of the device. In such prior art devices, the layer thickness is such that it is not the size limiting step, as far as the crystallographic size is concerned.        
In such structures the titanium and platinum layers 4, 5 are typically a few tens of nm in thickness, whereas the gold layer 6 is generally up to several hundreds of nm thick. The layers are usually deposited either by evaporation or by sputtering. A further even thicker layer of electro-plated gold (not shown) may be formed on top of the gold layer 6. A further metallization (not shown) is typically also provided on the semiconductor substrate 2 as a secondary electrode, and may be on a surface opposite to the metallization structure.
The application of the metal layers to the semiconductor can cause stress in the semiconductor. For many applications this stress is generally relatively unimportant. However, in optical waveguides such stress can produce undesirable optical effects.
It is known that thick metallizations produce built-in stress, at the start-of-life (SOL) of the semiconductor, and efforts have been made to produce metal layer systems with low built-in SOL stress. For example, the paper “The effect of process-induced stress in InP/InGaAsP weakly confined waveguides”, R. Rousina-Webb et al., from Optoelectronic Interconnects VII: Photonics Packaging and Integration II, Proceedings of SPIE Vol 3952 (2000), discusses broadening and mode splitting of the near-field optical output of a waveguide caused by built-in stress from a Ti:Pt:Au electrode provided on a waveguide ridge of an optoelectronic device, and goes on to disclose an approach to reduce this effect. The paper discloses that SOL stress is affected by the annealing stage of post-deposition processing, although the possible mechanisms causing this are not discussed. The proposed solution is the use of an additional metal layer (e.g. tungsten) to produce opposite stress to balance the innate tensile stress of the other layers (i.e. Ti, Pt and Au). However, in high performance devices such solutions are only suitable for devices that are operated at a single constant temperature, due to the effects of differential thermal expansion. Further such devices may be liable to irreversible changes due to plastic deformation of the electrode, as a result of exposure to extremes of temperature, even in storage.
Besides lateral mode guiding problems, stress in optoelectronic devices causes other undesirable effects such as uncontrolled refractive index change, which affects the optical path length of a waveguide. Three examples of problems produced by uncontrolled refractive index change in semiconductor optoelectronic devices are:                Refractive index change in a distributed Bragg reflector laser (DBR) changes the optical cavity length, changing the lasing wavelength and potentially inducing mode hopping.        Refractive index change in a distributed feedback laser (DFB), and also in the grating section of a DBR laser changes the effective pitch of the gratings and thus the selection of the lasing wavelength. Such a change will also potentially induce mode-hopping in DBR lasers.        Mach-Zehnder interferometer modulators (MZ) work by splitting a signal through two waveguide arms of controllable optical path length and recombining the signal to produce controlled optical interference. Refractive index changes affect the optical path length difference of the two arms, affecting the output power from the recombiner, with a deleterious effect on the extinction ratio (e.g. optical output power ratio of 1s and 0s in binary data modulation).        
It is known that optoelectronic devices are temperature sensitive, and that temperature dependence of built-in stress is a contributory factor. To overcome the problems of temperature dependant stress, optoelectronic components are typically mounted on thermoelectric coolers (TEC, also known as Peltier coolers) that enable the chip to be maintained at a substantially constant temperature.
However, an additional problem has now been identified, which is that metallizations can change due to aging effects during a product lifetime. Metallizations are polycrystalline layers, and the crystal boundaries move over time, by a process known as creep. Gold, in particular, is very soft, and stresses in the material—produced during deposition or subsequent temperature cycling—relax over long periods of time. This can produce changes in built-in stress, typically by relaxation of stress, which changes the optical performance of the optoelectronic device. This creep induced problem is not overcome by the use of temperature stabilization, e.g. by a TEC. Furthermore the SOL stress optimisation discussed by Rousina-Webb et al. above does not fully overcome the problem of changes in stress caused by creep.
There is a need, therefore, for a metallization system which overcomes, or at least alleviates, these problems.